Recently, liquid crystal display devices have been widely used for personal computers, word processors, amusement apparatuses, and television sets. However, unlike impulse-type display devices such as CRTs in which display light is instant, liquid crystal display devices are hold-type displays in which display light changes serially with time, and therefore have lower response time. Consequently, the liquid crystal display devices have a problem such that image deterioration such as motion blurring occurs particularly in displaying moving images. For that reason, methods for improving response characteristics in display have been discussed so as to display moving images with higher quality.
As a kind of the methods, there is provided a method in which a hold-type display device such as a liquid crystal display device is caused to have false impulse display characteristics, that is, display light is caused to be instant or intermittent as with a CRT.
In order that a liquid crystal display device has impulse response characteristics, known citation 1 (Japanese Unexamined Patent Publication No. 66918/2003 (Tokukai 2003-66918; published on Mar. 5, 2003) (corresponding to US20030058229A1) discloses a display device which operates in such a manner that: blanking data is inserted between sets of video data each corresponding to one frame period, and video data and blanking data are displayed alternately in one frame period. This allows for preventing deterioration in image quality due to motion blurring while preventing the display device from having a larger or more complex structure.
To be specific, as illustrated in FIG. 22, the display device of the known citation 1 includes: a plural-scanning data generating circuit 102 for inserting blanking data between sets of video data each corresponding to one frame period, the video data being supplied from a video signal source 101; a plural-scanning timing generating circuit 103 for generating timing for driving a gate line; and a display element array 106.
As illustrated in FIG. 23, in a scanning signal generated by the display device, a frame period 301 is equally divided into an image scanning period 302 and a blanking scanning period 303. That is, A gate line is selected twice in one frame period. In the image scanning period 302, signals are written in two lines simultaneously and two lines are subjected to interlaced scanning. That is, G1 and G2 are selected and video signals are written in G1 and G2 simultaneously, and then G3 and G4 are selected and next video signals are written in G3 and G4 simultaneously. Thereafter, in the same way, blanking data is written in two lines simultaneously and two lines are subjected to interlaced scanning.
At that time, as illustrated in FIG. 24, in a pixel of the display array, a video signal is written in an image writing period 402 of a frame, period 401 and blanking data nearer to a common level than a gradation voltage of an image is written in a blanking writing period 403. That is, a video signal indicated by a source waveform 406 is written in a selection period indicated by a gate driving waveform 405 in the image writing period 402, and transmittance increases as indicated by an optical response waveform 409. A canceling signal indicated by the source waveform 406 is written in a selection period indicated by the gate driving waveform 405 in the blanking writing period 403, and transmittance decreases as indicated by the optical response waveform 409.
The driving method allows for a display as illustrated in FIG. 25(a). That is, an original image 801 from the video signal source 101 is compressed by the plural-scanning data generating circuit 102 into one half in a longitudinal direction, and an ineffective image is added to the other half. As illustrated in FIG. 25(b), if the image is written with timing generated by the plural-scanning timing generating circuit 103, which timing allows for signals to be simultaneously written in two lines and for two lines to be subjected to interlaced scanning as described above, then video data and blanking data are displayed in one frame, so that image response and black response are repeated. This allows the display device to have impulse-type display characteristics, allowing for preventing deterioration in image quality due to motion blurring.
Further, known citation 1 discloses a method in which an original image is compressed into a quarter and a frame period is divided into four equal parts. In this case, a high-speed-liquid-crystal-response image (image obtained by emphasizing the original image) is generated by use of a high speed response filter so as to have higher response, and is written in a quarter of the frame period, and an image is written in a next quarter of the frame period, and blanking data is written in a remaining half of the frame period. This allows for further higher response.
Further, known citation 1 discloses that: when the same kind of scanning as the above is performed for scanning for one line, a writing time for one line is shortened so as to be approximately a half.
Further, known citation 2 (Japanese Unexamined Patent Publication No. 149132/2002 (Tokukai 2002-149132; published on May 24, 2002) discloses a method in which a canceling signal is written before each sub-frame period and a video signal is corrected so that a larger difference is provided between a canceling signal level and the corrected video signal. This allows for increasing a response speed of a liquid crystal, resulting in higher image quality in displaying moving images.
However, although the display device disclosed in known citation 1 allows for rapid rising from a black level of an optical response waveform by using a high-speed-liquid-crystal-response image, the display device has a problem that if blanking data is not completely written, then an exact image is not displayed.
To be specific, in a case where blanking data is not completely written, a voltage application indicated by a broken line waveform in an upper part of FIG. 26 causes an optical response indicated by a broken line waveform in a lower part of FIG. 26. Note that, in FIG. 26, a polarity is inverted when a transition from a voltage corresponding to a video signal to V0H corresponding to a canceling signal is performed (in FIG. 26, out of voltages corresponding to transmittance Tx, a voltage in a + driving is referred to as VxH and a voltage in a − driving is referred to as VxL).
To be specific, the display device of known citation 1 in which blanking data is displayed is premised on that: transmittance of a liquid crystal becomes Ta in accordance with a voltage VaL corresponding to a previous video signal in a video signal scanning period 32a and then the transmittance becomes T0 (steady state) in a canceling signal scanning period 33a, as indicated by a full line. Therefore, if a voltage VxH corresponding to a current video signal is supplied in the video signal scanning period 32b, then a voltage Vx′H is applied so that transmittance of a liquid crystal changes from T0 to Tx corresponding to a video signal Vx. However, in reality, the liquid crystal has a slow response. Consequently, as indicated by a broken line, a waveform indicative of the transmittance of the liquid crystal does not reach T0 in the cancel signal scanning period (the waveform reaches T0′ higher than T0), and in the video signal scanning period 32b, the transmittance of the liquid crystal reaches Tx″ higher than Tx which is target transmittance.
Further, at that time, even if a voltage V0 of the canceling signal is constant (V0H or V0L is applied according to inversion of polarity), transmittance T0′ of the liquid crystal at a time when a next signal begins to be written varies depending on a video signal Va of a previous frame period. Consequently, a voltage Vx′ for giving transmittance Tx in accordance with a previous video signal Vx also varies. Therefore, with a conventional method for applying a certain voltage in accordance with the video signal Vx, it is impossible to exactly display a gradation indicated by a supplied video signal. Consequently, it is impossible to display moving images with high quality.
Further, the liquid crystal display device disclosed in known citation 2 sets a video signal on the premise that writing a canceling signal would homogenize initial states of a liquid crystal in a frame period. The liquid crystal display device is not premised on that: because of a slow response of a liquid crystal, applying a voltage corresponding to a canceling signal would not allow for homogeneous transmittance which is desired. As described above, if a liquid crystal in an initial state is not in a uniformed state, then an applied voltage deviates from a voltage to cause target transmittance, so that an image true to an original video signal is not displayed.